Perfluorocarbon copolymers offer generally substantial resistance to corrosive effects of aggressive chemicals and have consequently found favor when used for fabrication of structures and or the coating of structures for use in contact with such aggressive chemicals. Additionally, perfluorocarbon copolymers have been provided with pendant ion exchange functionality, and have found considerable utility for a variety of tasks that can include functioning as a membrane in separating anode and cathode compartments within an electrochemical cell, and functioning, in powder form, as a catalyst for a variety of desirable organic reactions.
One perfluorocarbon copolymer finding particular acceptance, hereinafter being the substance referred to by the term perfluorocarbon copolymer, is generally a copolymer of two monomers with one monomer being selected from a group including vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkylvinyl ether), tetrafluoroethylene and mixtures thereof.
The second monomer is selected from a group of monomers usually including or derived from SO.sub.2 F, that is a sulfonyl fluoride group, or a group including or derived from COF, that is carbonyl fluoride. Examples of such second monomers can be generically represented by the formula CF.sub.2 .dbd.CFR.sub.1 SO.sub.2 F or CF.sub.2 .dbd.CFR.sub.1 COF. R.sub.1 in the generic formula is a bifunctional perfluorinated radical comprising generally 1 to 8 carbon atoms but occasionally as many as 25 carbon atoms. One restraint upon the generic formula is a general requirement for the presence of at least one fluorine atom on the carbon atom adjacent the SO.sub.2 F or COF, particularly where the functional group exists as the (SO.sub.2 NH).sub.m Q form. In this form, Q can be hydrogen or an or alkaline earth metal cation and m is the valence of Q. The R.sub.1 generic formula portion can be of any suitable or conventional configuration, but it has been found preferably that the vinyl radical comonomer join the R.sub.1 group through an ether linkage.
Typical sulfonyl fluoride containing monomers are set forth in U.S. Pat. Nos. 3,282,875; 3,041,317; 3,560,568; 3,718,627 and methods of preparation of intermediate perfluorocarbon copolymers are set forth in U.S. Pat. Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583. These perfluorocarbons generally have pendant SO.sub.2 F based functional groups. Typical methyl carboxylate containing monomers are set forth in U.S. Pat. No. 4,349,422. Perfluorocarbon copolymers containing perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) comonomer and/or methyl carboxylate monomers such as perfluoro(4,7-dioxa-5-methyl-8-nonenoate) have found particular acceptance in fabricating membranes for use in separating anode and cathode compartments. Conversion of sulfonyl fluoride groups to carboxylate groups is discussed in U.S. Pat. No. 4,151,053. The cationic exchange capability of the copolymeric perfluorocarbon having pendant sulfonyl and/or carbonyl functional groups is activated by saponification with a suitable or conventional compound such a strong caustic.
Often it is desired that perfluorocarbon copolymer be of an equivalent weight of between at least about 900 and about 1500 to provide a structure or catalyst having desirable performance characteristics. From time to time there the perfluorocarbon copolymer is to perform as a catalyst, it is desirable that perfluorocarbon copolymer be available in quite finely divided particulate form, the particles having a size distribution in a range of from about 10 to about 100 microns. Particles in such a size range may be generated by a procedure commonly known as cryogenic grinding, wherein the perfluorocarbon copolymer is subjected to a grinding or ball milling procedure at a quite low temperature (cryogenic). These cryogenic procedures can be quite expensive.
In another proposal, the perfluorocarbon copolymer can be prepared in finely divided particulate form as the copolymer is formed during polymerization in a suspension polymerization process. One difficulty with such forming techniques is that particles generated during copolymerization tend to be rather larger than may be desired. For example, Nafion.RTM. 511, a duPont particulate product, upon analysis of a sample was
4.3% less than 44 microns PA1 8.1% greater than 44 but less than 88 microns PA1 26.1% greater than 88 but less than 212 microns PA1 57.1% greater than 212 but less than 595 microns PA1 4.4% greater than 595 microns
Were a distribution of smaller particles available a desirably larger ratio of particles surface, that is catalytic surface to volume could be realized.
The use of alcohols to solvate particularly low equivalent weight perfluorocarbon copolymers is known. However, as yet, proposals for solvation of perfluorocarbon copolymer of equivalent weights in excess of about 900 have not proven generally satisfactory. Dissatisfaction has been at least partly attributable to a lack of suitable techniques for dispersing or solvating in part these higher equivalent weight perfluorocarbon copolymers.
At more elevated equivalent weights, perfluorocarbon copolymer contains PTFE (polytetrafluoroethylene) like crystallinity. As is well known in polymer chemistry, once crystalline polymer material commences appearing in a copolymer, dissolution becomes substantially more difficult. While temperature elevation is a frequently useful tool in such situations, with perfluorocarbon copolymers having pendant cation exchange functional groups, the usefulness of temperature elevation may be substantially limited. Known solvents for low equivalent weight copolymeric perfluorocarbons generally are possessed of a relatively low boiling point limiting the extent to which temperature elevation can be employed. In addition perfluorocarbon copolymer demonstrates a temperature degradation characteristic beginning to be significant at between about 250.degree. C. and 300.degree. C. or less.
For perfluorocarbon copolymers having pendant sulfonyl fluoride functionality, crystallized PTFE-like material begins to appear in the copolymer at between about an equivalent weight of 910 and 1050. Further, as described by Yeo in "Solubility Parameter of Perfluoro-sulfonated Polymer", perfluorocarbon solubility apparently is a function of the equivalent weight, becoming of substantial consideration above an equivalent weight of between about 910 and 1050 for sulfonyl fluoride functionality. Therefore solvents functioning upon lower equivalent weight material would appear not likely to function adequately at more elevated equivalent weight. Other articles such as: Seko et al "Perfluorocarboxylic Acid Membrane and Membrane Chlor-alkali Process Developed by Asahi Chemical Industry", Gierke et al "Morphology of Perfluorosulfonated Membrane Products", and Hashimoto et al "Structure of Sulfonated and Carboxylated Perfluorinated Ionomer Membranes", collected in Eisenberg et al "Perfluorinated Ionomer Membranes", Yomigama et al "Paper at No. 5 Caustic Soda Technical Forum, Kyoto Japan 11/81" and Starkweather "Crystallinity in Perfluorosulfonic Acid Ionomers and Related Polymers" further describe this phenomenon.
Particularly where particulate perfluorocarbon copolymer is to be employed as a catalyst, for example as an acid catalyst, relatively small particles tending to optimize the ratio of surface area to volume of perfluorocarbon copolymer are advantageous. A relatively inexpensive method for forming such particles having a relatively large selection of pendant functional groups available would likely find utility in the manufacture of these acid catalysts.